Research On Imbalance Technologies Of Thermal Management In Multilevel Converters

To meet further improvements of the national energy saving and emission reduction demand,new requirements in industrial production,aerospace,transportation,high voltage flexible transmission and other occasions are put forward for higher voltage level of the converters.The contradiction between the voltage levels of power devices and converters can be solved by multilevel converters,of which the output waveforms are good and the fault tolerance is high.For multilevel converters,the number of power devices is greatly increased,and the number of output switching states in the multilevel converter is increased cubically with the number of levels.And there are a lot of redundant switching state combinations.The imbalance degree of device power losses increases accordingly.The originally methods with ideal periodic sinusoidal pulse width modulation strategy(with ideal sinusoidal load current and voltage,stabilities of system parameters,without fluctuation of DC voltage,at fixed switching frequency periodic modulation strategy,and so on)for analysis and calculation of the converter power losses are no longer applicable.In this paper,IGBTs and anti-parallel diodes are taken as the typical power devices.Considering the influence of the junction temperature and other factors,mathematical fitting method based on the data is adopted.Combining with the topology and modulation strategy of the converter,a universal power loss calculation model of IGBTs and anti-parallel diodes is established in software MATLAB/Simulink.The parameters for device loss calculation are obtained and power loss calculations of converters are accomplished.In this model,it is equivalent to calculate the power losses of each device according to the discrete time state(on state and off state),and the state before.Thus each device in action(keep on state,off state,turn open or shut off)at the start of the discrete time is judged and corresponding energy loss is calculated.This method is simpler,and has good portability for new topologies and modulation algorithms.Loss calculation helps multilevel converters for its cost reduction,device lifetime and reliability improvements.Model predictive control is a new modulation algorithm of great concern in recent years.A rapid prediction model for cascade H-bridge STATCOM system is studied.Combined with the loss calculation model before,quantitative analysis about influences on device power loss are made with the change of grid voltage,control time and the weight coefficient of the switching loss.The reference working temperature range and the typical values are provided for thermal design.The IF(imbalance factor)of the modulation algorithm is used as an example to evaluate the unbalance degree of device power loss.Finally,in order to solve the imbalance problem of device power losses in multilevel converters,the application of the thermoelectric coolers to the multilevel converter is studied.The lifetime of a power device is related to the amplitude of device junction temperature,the range of junction temperature,the average junction temperature,the diameter of module bonding wire,device current and blocking voltage and other factors.Usually,there is only one cooling method in a multilevel converter.Uneven power losses of devices will cause uneven temperature distribution,which may lead to waste costs,lifetime reduction and reliability issues.Thermoelectric coolers are easy to control,maintenance free,which can be used as a supplementary cooling method to reduce power junction temperature fluctuations,lower power device junction temperature.Alternatively,even when the device is overheating,it can be treated as a supplementary method of forced cooling.In depth analysis of the thermoelectric cooling device on energy exchange mechanism,the mathematical model of the thermoelectric cooling devices is established more accurately,which is helpful for accurate temperature control.The results will lay a foundation for the development and the improvement of control power for thermoelectric cooling devices and multilevel converter applications with thermoelectric coolers.